Tumor metastasis causes a high mortality of patients with triple-negative breast cancer (TNBC), but its underlying mechanism remains unknown. Emerging studies showed that dysregulated gene transcription mediates TNBC malignancy and tumor microenvironment is one of the critical regulators of gene dysregulation. Hypoxia-inducible factors (HIFs), mainly HIF-1 and HIF-2, are the master transcriptional regulators in response to hypoxia, a common feature of the tumor microenvironment, and activate their downstream target genes to regulate many key processes in cancer biology, including angiogenesis, extracellular matrix remodeling, cell migration/invasion, leading to TNBC progression and metastasis. Thus, understanding fundamental regulation of HIF transcriptional activity would uncover the mechanism of TNBC progression and metastasis, which may lead to discovery of therapeutic vulnerabilities in TNBC. To identify the novel regulator that controls HIF activity, we screened 720 epigenetic regulators and found that SAP30 is induced by HIF-1 and HIF-2, and in turn interacts with the alpha subunit of HIF-1 and HIF-2 to enhances their target gene expression in TNBC cells. SAP30 is highly and selectively expressed in human TNBC, and high levels of SAP30 are positively correlated with poor survival of these patients. Genetic deletion of SAP30 blocks TNBC growth and lung metastasis in xenograft mice. These exciting results suggest that SAP30 may act on HIFs-mediated gene dysregulation to promote TNBC progression and metastasis. The central goal of the current R01 project is to dissect the detailed mechanisms by which SAP30 increases HIF activation and TNBC progression and metastasis. Three Specific Aims are proposed to address this goal. In Aim 1, we will define the underlying mechanism of SAP30 upregulation in TNBC by using human TNBC cell lines and tissues. In Aim 2, we will decipher the regulatory mechanism of SAP30-mediated HIF activation in TNBC. In Aim 3, we will dissect the molecular mechanisms of SAP30-dependent TNBC growth and metastasis using SAP30 knockout mice we generated as well as human TNBC xenograft mice. The goal will be achieved with great support from our outstanding collaborators on campus at UT Southwestern. The successful completion of this project will provide new insights into the fundamental molecular mechanism underlying TNBC progression and metastasis, and may uncover SAP30 as a therapeutic vulnerability in TNBC.